HBD (unknown acronym meaning) is a disease from the Mega Man Battle Network series. Two known cases of it are revealed by the series: Hub Hikari and Mamoru. The former "died" (albeit survived in a digital form), and the latter survived thanks to advanced technology. Symptoms include severe chest pain, and being unable to breathe. It is so rare that the only place they could've tested Mamoru for HBD was at Seaside Hospital. There are no curing medications. As a bacterium or virus could not have caused Mamoru or Hub Hikari to have such severe chest pain, it is likely a hereditary disease/disorder. The only treatment and possible cure is surgery, but the procedure's name is never revealed.
- The meaning of "HBD" was never revealed, but there are speculations that HBD stands for "Heart Beat Disorder" or "Heart Broken Disease".